Restore thumbnails in Windows File Explorer?
The answer courtesy of fmb501 at blenderartists (thread here) :


  This appears to be a bug in the Blender installer, but the fix is
  fairly simple.
  
  Open the Windows icon in the bottom left corner of your computer
  display and search for "command prompt" 
  
  Right click on the result and select "run as administrator."
  
  There will be a small window with a black background and a blinking
  cursor.
  
  Type "chdir c:\program files\blender foundation\blender" and press
  Enter (chdir is the command for changing directories with the command
  prompt)
  
  You should now be at that directory.
  
  Type "blender -R" and press enter. You should receive a message
  indicating that the registration has been successful.
  
  Close any Windows Explorer windows and then open Windows Explorer
  again.
  
  Navigate to a directory that has your blend files and you should now
  see thumbnails for all of your blend files. If there some blend files
  that don't have thumbnails, it's probably an old file before the
  thumbnails were added. Just open and save it again and you should see
  the thumbnail.
  
  I'm not sure why Blender Foundation hasn't fixed this instalation
  problem by now. I'm wondering if you have to install blender as an
  administrator to get it to regiser the thumbnail dll.
  
  Anyway, until it gets fixed, you will likely have to register as
  described above with every new installation.